The present invention relates to improved water-swellable, cross-linked polymers and an improved method of thickening aqueous media therewith.
Techniques have been disclosed whereby high molecular weight linear polymers have been made utilizing a water-in-oil emulsion polymerization process in which a water-soluble monomer is emulsified in an oil phase and polymerized therein. Such a process is revealed in U.S. Pat. No. 3,284,393. Inverse suspension polymerization of water-soluble unsaturated monomers is taught in U.S. Pat. No. 2,982,749 wherein linear polymer beads ranging in size between 10 microns and 2 millimeters in diameter are formed. The polymeric products of these processes have been found to be useful as binders in coating compositions, and as flocculating, suspending or thickening agents.
These processes produce linear polymers which have virtually no gel strength. Gel strength refers to that property of the water-swollen polymers which resists viscosity change as the result of mechanical working or milling.
U.S. Pat. No. 3,247,171 reveals a process which produces a water-swellable polymer with high gel capacity obtained by the use of controlled cross-linking and hydrolysis. This process produces bulk gel polymer through the use of bulk solution polymerization. The resulting polymer may be dried and ground to a fine powder. However, the particles of polymer so prepared are still of substantial size so that aqueous slurries prepared therefrom are shear sensitive. Consequently in applications such as paper coating where the use of rolls and doctor blades produces a shear of from 100,000 to 200,000 reciprocal seconds, the polymer particles are degraded and thereby lose much of their thickening and water-holding properties. Similarly, where colloid mills are used such as in the making of paints, the linear polymers made according to the first two mentioned patents and the polymer particles of the last mentioned patent must be added to the paint after the step employing the colloid mills or degradation of the polymers will occur.